Btrfs: add WARN_ONCE to detect unexpected error from merge_extent_mapping

This is a subtle case, so in order to understand the problem, it'd be good
to know the content of existing and em when any error occurs.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 6fe8b14e11cf0285152ff00f5fcf722dbc57fc23..914662428dbd434f4ffc7c7a84b0720a1308e0bb 100644 (file)
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -562,17 +562,24 @@ int btrfs_add_extent_mapping(struct extent_map_tree *em_tree,
                        *em_in = existing;
                        ret = 0;
                } else {
+                       u64 orig_start = em->start;
+                       u64 orig_len = em->len;
+
                        /*
                         * The existing extent map is the one nearest to
                         * the [start, start + len) range which overlaps
                         */
                        ret = merge_extent_mapping(em_tree, existing,
                                                   em, start);
-                       free_extent_map(existing);
                        if (ret) {
                                free_extent_map(em);
                                *em_in = NULL;
+                               WARN_ONCE(ret,
+"unexpected error %d: merge existing(start %llu len %llu) with em(start %llu len %llu)\n",
+                                         ret, existing->start, existing->len,
+                                         orig_start, orig_len);
                        }
+                       free_extent_map(existing);
                }
        }